Assembly for pressure control when drilling and method to control pressure when drilling in a formation with unpredictable high formation pressure

ABSTRACT

Assembly for pressure control when drilling underground to locate or produce hydrocarbons; whereby the drilling is done by means of a drill string with fluid circulation down through the drill string and up through an annulus between the drill string and the open hole or casing, as the underground in the open hole may comprise one or both of: areas with formation strength which is unable to withstand the pressure from the fluid circulation without fracturing and areas with formation pressure which cannot be combined with each other or the fluid circulation pressure without the fluid unintentionally leaking in or out from the above-mentioned areas. The assembly is distinguished in that onto the drill string a bottomhole assembly is fitted, comprising: a reversible and expandable packer fitted near to the lower end of the drill string to seal the annulus, controllable from the surface a measuring device for measuring the pressure inside the annulus both above and below the packer, with means to simultaneously transfer the data to the surface, a circulation valve fitted above the packer and controllable from the surface in order to open or close off the communication of fluids between the inside of the drill string and the annulus, a control valve (closing valve) fitted below the circulation valve and controllable from the surface, to open and close off the communication of fluids through the drill string, and a connecting sub located above the packer and controllable from the surface for connecting and un-connecting the drill string.

AREA OF THE INVENTION

The present invention relates to drilling to locate or produce fluid hydrocarbons. More precisely, the invention relates to an assembly for pressure control when drilling underground to locate or produce hydrocarbon and a method for which the assembly is used. The assembly is generally of use in situation where the pressure from the well cannot be controlled solely with the help of circulating the drilling fluid and by altering the fluid density, as in when drilling in a formation with unpredictable high formation pressure, whereby the drilling fluid density cannot be adequately increased to control the high formation pressure without fracturing the formation at a more shallower level of the open part of the well.

BACKGROUND OF THE INVENTION AND PRIOR ART

When drilling in wells to locate or produce oil and gas, a drill bit, which is attached to the end of a drill string comprising joined pipe sections, is used. The drill bit is rotated against the formation when drilling, whilst the drilling fluids or mud is pumped into the drill string. The drilling fluids flow down the drill string, out through the drill bit and up towards the surface through a annulus created between the drill string and the open hole; or for the other parts of the well, between the drill string and the casing, which is cemented to the formation. The drill bit is usually rotated by rotating the drill string from the surface, whilst the drill string is suitably stretched, with adequate weight applied to the formation by the drill bit. The circulating drilling fluid serve as greasing and aids the removal of the drill solids (cuttings) from the borehole as well as means of controlling the pressure on the formation being drilled through. It is vital that the density of the drilling fluid is such so that the pressure to the formation can be controlled, so that the formation fluid does not uncontrollably stream into the well.

Should a formation layer with unpredictable high formation pressure be met, then the density of the drilling fluids in the well should be increased in order to control the unpredictable high formation pressure. This can be achieved with the use of equipment to close off the annulus between the drill string and the formation at the bottom at a level just above the formation with the unpredictable high pressure, and combined with opening a way for fluid communication from the drill string to the annulus above the closure of the annulus, and equipment for closing off the fluid communication through the drill string to or from the annulus at level below the closure of the annulus. The closure of the annulus is typically done by means of a packer, which is an inflatable device that can be inflated reversibly by means of hydraulics or in some other way. The fluid communication between the drill string and the annulus above the packer is typically achieved by means of a circulation valve, which can be opened to provide fluid communication between the drill string and the annulus. At lower level in the drill string, a closing valve may be installed. By means of the mentioned tools, it is possible to circulate drilling fluid with higher density in the annulus, above the safety valve and therefore be able to balance the pressure from the drilling fluid against the unpredictable high formation pressure. Once an overbalance of the fluids against the formation pressure is achieved, the circulation valve may be closed off, the packer can be contracted so that the annulus is opened, the closing valve may be opened and the drilling continued.

A down-hole blow-out preventer is described together with a user guide in the patent publication U.S. Pat. No. 4,588,035. More specifically, there is a description of a device used to close off the drill string at its lower end and to close off the annulus between the drill string and the wall of the borehole by means of an inflatable packer and the subsequent balancing of drilling fluid circulation. The packer may be operated when a predetermined difference in pressure exists between the annulus and the drilling fluid column inside the tool. Other similar tools with similar functionalities are described in the U.S. Pat. No. 4,367,794, 5,404,953, 4,712,613 and 3,853,177.

The above mentioned devises for pressure control have proven inadequate in providing simultaneous data to the surface in order to control the operation most efficiently and the mechanical devices are lacking the necessary reliability. Furthermore, it is not always possible to resume the drilling as intended because it is not always possible to control the formation pressure by changing the mud density. At the end section of the borehole, there is still no casing or casing extension cemented to the formation wall and by drilling this so-called open part of the borehole is extended; this means the section where the formation lays bare towards the borehole. It is not always possible to adequately increase the drilling fluid density in order to balance out the formation pressure without fracturing the formation at a more shallow level, in the open part of the well. This will result into the drilling fluid leaking into the fractured formation instead of balancing out the formation pressure. Further, the drilling fluid can leak from one formation to another in the open part of the well. Drilling into a formation with unpredictable low formation strength may make it impossible to sufficiently reduce mud density to avoid loss of mud to the mentioned formation. The above-mentioned problems may arise in particular when drilling in formations of the changeable type and/or with changeable characteristics. There is a demand to solve the above-mentioned problems.

SUMMARY OF THE INVENTION

With the present invention the above-mentioned problems are solved by providing an assembly for pressure control when drilling underground to locate or produce hydrocarbons; whereby the drilling is done by means of a drill string with fluid circulation down through the drill string and up through an annulus between the drill string and the open hole or casing, as the underground in the open hole may comprise one or both of: areas with formation strength which is unable to withstand the pressure from the fluid circulation without fracturing and areas with formation pressure which cannot be combined with each other or the fluid circulation pressure without the fluid unintentionally leaking in or out from the above-mentioned areas. The assembly is distinguished in that onto the drill string a bottomhole assembly is fitted, comprising:

-   -   a reversible and expandable packer fitted near to the lower end         of the drill string to seal the annulus, controllable from the         surface     -   a measuring device for measuring the pressure inside the annulus         both above and below the packer, with means to simultaneously         transfer the data to the surface,     -   a circulation valve fitted above the packer and controllable         from the surface in order to open or close off the communication         of fluids between the inside of the drill string and the         annulus,     -   a control valve (closing valve) fitted below the circulation         valve and controllable from the surface, to open and close off         the communication of fluids through the drill string, and a         connecting sub located above the packer and controllable from         the surface for     -   connecting and un-connecting the drill string.

The open hole means the lower section of the well where a casing or casing extension still has not yet been cemented. Casing also refers to casing extension. Controllable from the surface, means that the equipment can be operated from the surface with the functionalities as defined.

Zones with formation density which are unable to absorb the pressure from the fluid circulation without fracturing; and zones with formation pressure which cannot combine with each other or fluid circulation pressure without the fluid unintentionally leaking into or out of the mentioned zones, are all meant to indicate situations where the well pressure cannot be controlled solely by means of traditional fluid circulation and increasing and decreasing of the fluid density. This may be when drilling onto zones with unpredictable high formation pressure, where the fluid density cannot be increased without fracturing the formation at a more shallow level of the open part of the well in the open hole whereby the fluid will leak into the fractured formation. These may be zones in the open hole with unpredictable low formation pressure or low formation density, to which zones the mud may be lost. And these may be zones with different pressure so that fluids stream from one zone to another by means of an underground blow-out.

The assembly advantageously comprise a communication/power module for transmitting power from the surface and for transferring measurement and control signals to and from the surface. The mentioned module also advantageously has a fitted battery capacity.

The measurement device for measuring the pressure inside the annulus above and below the packer, together with simultaneous data transfer to the surface, also advantageously comprise a device for temperature measurement above and below the packer, and further measurement devices, such as a device for measuring the difference in pressure over the packer; and devices for measuring the flow rate and the composition inside the annulus. The measurement device is advantageously fitted in one place, for example above the packer, but with fluid columns from the annulus above and below the packer to respective pressure measuring devices.

The invention also provides a method for pressure control when drilling in a formation of unpredictable high formation pressure, whereby the density of the drilling fluid cannot adequately be increased to control the formation pressure without fracturing the formation at a more shallow level in the open part of the well, whereby the drilling is carried out by using a drill string with drilling fluid circulation down through the drill string and up through the annulus between the drill string and the open hole or casing; the drill string comprising an assembly in accordance with claim 1, whereby the annulus closes off by means of a packer in the composition, the fluid communication through the drill string is closed off by means of a control valve on the drill string; drilling fluid circulation is carried out in the borehole above the packer by opening a circulation valve on the drill string and then circulate the drilling fluid with as much density as feasibly possible without fracturing the formation on the open part of the borehole.

The method is distinguished by:

-   -   un-connecting the drill string at the connecting sub fitted onto         the assembly above the packer and the control valve,     -   a high viscosity pill or gel pill is placed in the borehole         straight above the connecting sub by using the drill string         before this is withdrawn,     -   casing is hoisted down and cemented to the open part of the         borehole,     -   a drill string is brought down the drilling hole in order to         drill out from the casing and to clean out from the drilling         hole as much as possible of the high viscosity pill down towards         the connecting sub,     -   an equipment string with a connecting sub or a fishing sub at         the end is guided down into the borehole,     -   circulation is then started for cleaning and balancing the         weight of the drilling mud in relation to the pressure below the         packer,     -   the equipment string is connected to the connecting sub, the         circulation valve is opened and the circulation continues         through the circulation valve, and     -   the control valve and the packer are opened during stable         conditions and the drilling is continued with the existing         equipment; or the drill string is extracted and replaced with a         new drill string with a drill bit and possibly a ream or         underream with a reduced diameter.

FIGURES

The present invention is illustrated by means of 10 figures, where:

FIG. 1 shows a standard drilling inside a well,

FIG. 2 shows drilling in a formation with unpredictable high pressure,

FIG. 3 shows the closing off of the annulus,

FIG. 4 shows the closing off of the drill string and pressure observation,

FIG. 5 shows the circulation of the drilling fluid to increase the drilling fluid density as much as feasibly possible,

FIG. 6 shows the drill string being unconnected and the placement of the high viscosity pill above the packer element,

FIG. 7 shows the introduction and cementing the casing in the open part of the hole,

FIG. 8 shows the drill string being connected,

FIG. 9 shows the action before the drilling can be continued with, and

FIG. 10 shows the continued drilling with the replaced drill string with reduced diameter to the ream or the underream.

DETAILED DESCRIPTION

Reference is made to FIG. 1, which illustrates standard drilling in a well, whereby the density of the drilling fluid is circulated through the drill string and back to the surface on the exterior of the drill string, which is sufficient to control the formation pressure inside the well; in other words, the density of the drilling fluid is controlling the formation pressure. The assembly used for the drilling, is in accordance with the current invention. More precisely, this comprises at the lower end of the drill string, from bottom and upwards: a (pilot) drill bit 1, a measuring device 2 to measure the pressure and temperature inside the annulus (below the packer 4), a reversible control or closing or isolation valve 3 (for example a ball valve) for opening and closing off the fluid communication through the drill string, a reversible and inflatable packer 4 for sealing the annulus, a caliper 5 for measuring of the borehole's diameter, a measuring device 6 to measure the pressure and temperature inside the annulus (above the packer 4), a communication/power module 7 for (possible) power transmission and transferring of measurement data and control signals to and from the surface, a circulation valve 8 (for example a flapper valve or sliding sleeve), which is controllable from the surface in order to open and close off the fluid communication between the inside of the drill sting and the annulus, a connecting sub 9 for connecting and un-connecting the drill string above the connecting sub, a ream 10 (alternatively an underreamer) in order to expand the diameter of the borehole. The equipment fitted to the lower end of the drill string 11 may comprise further units in connection with a standard drilling operation. However, for clarity sake, such further units are not described as it is assumed that these units are familiar to people within this field. The drilling illustrated in FIG. 1, is carried out in a tight formation 12, in this case shale. In front of the drill bit there is however, a permeable formation 13; in this case, sandstone; and FIG. 2 shows that the drilling is continued through the sandstone formation 13, which in this case is a formation with an unpredictable much higher pressure than the pressure the fluid density is controlling at the actual depth; so that the formation fluid from the sandstone formation streams into the well, known as “kick” or a “well kick”. FIG. 2 shows the mentioned inward stream, which is indicated with arrows 14 of a lighter colour than the arrows 15, which indicate the fluid circulation. FIG. 3 shows that the packer 4 is being inflated in order to close off the fluid communication inside the annulus on the exterior of the drill string. In FIG. 3, it is clearer how the packer is inflated against the formation wall in order to close off the annulus. FIG. 4 illustrates that the control valve 3 fitted to the drill string is closed off to avoid fluid communication from the high-pressure formation into the drill string. In a situation like this, the pressure gauges 2 and 6 are used to measure the pressure on top of and below the packer, in order to provide the necessary information regarding adequate fluid density to control the unpredictable high pressure. FIG. 5 shows the opening of the circulation valve 8 and the circulation inside the well with fluid with as much high density as feasibly possible without fracturing the formation above the packer. The arrows 15 indicate this circulation. FIG. 6 shows the method of connecting the drill string from the connecting sub above the packer and placing a high viscosity pill (alternatively a gel pill) above the packer to avoid dirt or cement around the packer and the connecting sub 9, as this may hinder subsequent operation. The drill string is connected from the bottom part of the connecting sub, for example by rotating in the opposite direction from the drilling rotation; or by means of another available method for connecting and unconnecting the drill string. The high viscosity pill 16 (alternatively the gel pill) is pumped into place through the drill string; whereas the drill string above the connecting sub 9 is extracted out of the borehole. The high viscosity pill 16 (alternatively the gel pill) is a fluid which can be pumped into place, with preferably a higher density than cement, in order for this to stabilise and protect the packer, the connecting sub and other remaining devices. FIG. 7 shows how the casing is inserted 17 and cemented 18 to the well wall. In this way the open part of the borehole, which is unable to withstand the necessary high fluid weight to control the well, is isolated. Once the casing is firmly cemented, a drill string is hoisted down into the hole to drill out from the casing to clean the borehole and as much as the high viscosity pill 16 as possible in order to prepare for subsequent operation (not illustrated). Then a device is inserted to first circulate and clean around the connection sub 9; the open hole above the packer is checked to verify that it can withstand adequate increase of the fluid weight, then the fluid weight is adequately increased for subsequent connecting of the equipment above the packer as illustrated by FIG. 8. FIG. 9 shows the monitoring of the well pressure above and below the packer 4, so if the fluid pressure above the packer 4 is adequate in relation to the pressure below the packer 4, then fluid communication with the formation below the packer may be established as the control valve is opened, the circulation valve is closed off and the packer contracts. The well is then in an adequate overbalance with the new drilling fluid density, and fluid may be circulated to clean the hole and secure the stability of the well before continuing the drilling with the same equipment. However, it may be more of an advantage to extract the remaining components and change these with newly serviced ones. The latter example is illustrated on FIG. 10, where the diameter of the drill bit is the same, whilst the ream, alternatively the underream has reduced diameter for the next hole section.

The signalling used with the current invention can generally be achieved by any familiar means, for example by means of pulse-echo through the drill string, electromagnetic signalling, fluid pulse techniques, connecting through the drill string or the compilation of cables. Transmission of power is normally achieved by any familiar means, for example by transmission of electrical power through cable or the drill string, placing a fluid operated turbine or battery capacity to the equipment. The signalling and the transmission of electrical power may be fitted to the each individual device, but is more advantageously collectively compiled through a power/communication module. It is further possible to combine the various components provided that the functionality as described in claim 1 is retained. Should all of the necessary power be achieved with a fluid operated turbine, it would then be necessary to maintain the fluid circulation until a well overbalance is achieved; for example by keeping the control valve closed and the circulation valve open while drilling fluid is circulated and drilling fluid density is increased until overbalance is achieved, after which the packer 4 is contracted and the control valve 3 opened so that the entire well is open for circulation.

The assembly according to the invention is generally usable in situations where the pressure of the well cannot be controlled solely by the means of drilling fluid circulation and adjustment of the drilling fluid density, as some of these situations are previously addressed. However, the assembly according to the invention is of course also usable in situations where the fluid circulation and adjustment of the density is enough to control the pressure of the well. The packer provides the opportunity to isolate part of the well and therefore simplify the control of the well. The pressure status above and below the packer over a period of time, may at an early stage provide information on the characteristics of the flow and possible well-kick since the pressure can be monitored in real time with the packer being open and after the packer is set. The assembly according to the invention may be combined with equipment for low scale well testing, for example by using the equipment described in the patent publication no. NO 3009396 to which reference is made. Furthermore, it is possible to fit several assemblies in accordance with the invention onto a drill string, or fit further packers, measuring devices and other tool components of the assembly. 

1. Assembly for pressure control when drilling underground to locate or produce hydrocarbons; whereby the drilling is done by means of a drill string with, fluid circulation down through the drill string and up through an annulus between the drill string and the open hole or casing, as the underground in the open hole may comprise one or both of: areas with formation strength which is unable to withstand the pressure from the fluid circulation without fracturing and areas with formation pressure which cannot be combined with each other or the fluid circulation pressure without the fluid unintentionally leaking in or out from the above-mentioned areas, characterized in that onto the drill string a bottomhole assembly is fitted, comprising: a reversible and expandable packer fitted near to the lower end of the drill string to seal the annulus, controllable from the surface a measuring device for measuring the pressure inside the annulus both above and below the packer, with means to simultaneously transfer the data to the surface, a circulation valve fitted above the packer and controllable from the surface in order to open or close off the communication of fluids between the inside of the drill string and the annulus, a control valve (closing valve) fitted below the circulation valve and controllable from the surface, to open and close off the communication of fluids through the drill string, and a connecting sub located above the packer and controllable from the surface for connecting and un-connecting the drill string.
 2. Assembly in accordance with claim 1, characterised by that it comprises a hole opener, an under ream or similar fitted within a suitable distance above the connecting sub.
 3. Assembly in accordance with claim 1, characterized in that it comprises a drill bit at the bottom end.
 4. Assembly in accordance with claim 1, characterized by comprising one caliper, for measuring the diameter of the hole, preferably by means of acoustic/ultrasonic signals.
 5. Assembly in accordance with claim 1, characterized by the measuring device also including a device for measuring the temperature, both above and below the packer.
 6. Method for pressure control when drilling in a formation of unpredictable high formation pressure, whereby the density of the drilling fluid cannot adequately be increased to control the formation pressure without fracturing the formation at a more shallow level in the open part of the well, whereby the drilling is carried out by using a drill string with drilling fluid circulation down through the drill string and up through the annulus between the drill string and the open hole or casing; the drill string comprising an assembly in accordance with claim 1, whereby the annulus closes off by means of a packer in the composition, the fluid communication through the drill string is closed off by means of a control valve on the drill string; drilling fluid circulation is carried out in the borehole above the packer by opening a circulation valve on the drill string and then circulate the drilling fluid with as much density as feasibly possible without fracturing the formation on the open part of the borehole, characterised by un-connecting the drill string at the connecting sub fitted onto the assembly above the packer and the control valve, a high viscosity pill or gel pill is placed in the borehole straight above the connecting sub by using the drill string before this is withdrawn, casing is hoisted down and cemented to the open part of the borehole, a drill string is brought down the drilling hole in order to drill out from the casing and to clean out from the drilling hole as much as possible of the high viscosity pill down towards the connecting sub, an equipment string with a connecting sub or a fishing sub at the end is guided down into the borehole, circulation is then started for cleaning and balancing the weight of the drilling mud in relation to the pressure below the packer, the equipment string is connected to the connecting sub, the circulation valve is opened and the circulation continues through the circulation valve, and the control valve and the packer are opened during stable conditions and the drilling is continued with the existing equipment; or the drill string is extracted and replaced with a new drill string with a drill bit and possibly a ream or underream with a reduced diameter.
 7. Method in accordance with claim 6, characterised in that the high viscosity pill has a high gel strength and preferably a higher density than cement mixture. 